Method and apparatus for tuning coaxial-line section resonators

ABSTRACT

In a coaxial-line section resonator having a short-circuited end, a short-circuited coupling loop is interposed in the space between the coaxial conductors and is mounted on a wall of the resonator for rotation from outside the resonator, whereby the resonant frequency of said resonator is adjusted in response to rotation of the loop.

0 United States Patent 1 [111 3,872,413

Schneider Mar. 18, 1975 [5 METHOD AND APPARATUS FOR TUNING 3,426,122 3134; 20W g ,4 1,8 l l 4 uarrera..... COAXIAL LINE SECTION RESOIiATORS 2,526,579 l0/l950 Ring 333/98 R [75] In entor: Guenter Schneider, c 2,752,576 6/1956 Hilliard et al 333/82 BT Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & Primary ExaminerJameS W. Lawrence Munich, Germany Assistant ExaminerWm. H. Punter Attorney, Agent, or Firm-Hill, Gross, Simpson, Van [22] Flled' 1973 Santen, Steadman, Chiara & Simpson [2]] Appl. N0.: 321,329

[30] Foreign Application Priority Data [57] ABSTRACT July 24, 1972 Germany 2236l68 In a coaxial-line section resonator having a shortcircuited end, a short-circuited coupling loop is inter- [52] US. Cl 333/82 B, 333/97 R posed in the space between the coaxial conductors [51 1 Int. Cl. H0lp 7/04, HOlp H00 and is mounted on a wall of the resonator for rotation [58] Field of Search 333/82 B, 82 BT, 97 R from outside the resonator, whereby the resonant frequency of said resonator is adjusted in response to ro- [56] References Cited tation of the loop.

UNITED STATES PATENTS 8 CI D 2.286.396 6/1942 Trevor 333/82 B 'awmg PATENTED I 3,872,413

sum 1 or 2 WT-MI METHOD AND APPARATUS FOR TUNING COAXIAL-LINE SECTION RESONATORS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coaxial-line resonator having the form of a coaxial-line section with one end short-circuited by an electrically conductive plate.

2. Prior Art Coaxial line resonators of the short-circuited type are used in the microwave frequency range as tunable resonant circuits. Such resonators are usually tuned by means of variable capacitance elements located near the free end of the inner conductor. When such elements are metallic in nature, special measures must be taken to insure that the tuning elements are in good electrical contact with one of the coaxial conductors. In another arrangement, the tuning element consists of dielectric material, and there is then a disadvantage resulting from a relatively large variation of the frequency of the resonator as a function of temperature changes.

It has also been known to vary the electrical length of the inner conductor, by means of a short circuiting piston, or by dividing the inner conductor into a stationary and a movable part, etc. This arrangement is effective to change the resonant frequency over a relatively wide range, but the mechanical construction for the adjustable conductor is relatively complicated and expensive. When such coaxial-line resonators are used in connection with a vehicle identification system, such as for railroad cars or the like, it is desirable to tune the resonators over a relatively narrow frequency range, so that the wide tuning range of complicated resonators, with adjustable inner conductor lengths, is not needed. It is however, desirable to provide a method and apparatus for tuning a coaxial-line section resonator over a narrow frequency range which is not subject to the disadvantages of the other known types of resonators.

SUMMARY OF THE INVENTION A principal object of the invention is to provide a simple means for tuning a coaxial-line resonator with-' out causing the resonator to be temperature sensitive.

Another object of the present invention is to provide a method and apparatus for tuning the resonator employing an adjustable means which provides a shift in the resonant frequency approximately proportional to the amount of movement of the adjustable means.

These and other objects and advantages of the present invention will become manifest upon an examination of the following description and the accompanying drawings.

In one embodiment of the present invention the resonator takes the form of a coaxial-line section having one end short-circuited by a conductive plate, with a rotatable field displacement body supported between the coaxial conductors and having a portion extending outside the coaxial-line section to permit rotation of the field displacement body, whereby the resonator may be tuned to different frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the FIG. 1 is a longitudinal cross-section of a coaxial-line resonator having a field displacement body mounted on the short-circuiting plate of the resonator;

FIG. 2 is a longitudinal cross-section of a coaxial-line resonator having a field displacement body on the outside wall of the resonator;

FIG. 3 is a side elevation of an alternative construction of a field displacement body;

FIG. 4 is an end elevation of the apparatus of FIG. 3;

FIG. 5 is a side elevation of yet another form of a field displacement body;

FIG. 6 is an end elevation of the apparatus illustrated in FIG. 5;

FIG. 7 is a plan view of the bottom of the apparatus illustrated in FIGS. 5 and 6; and

FIG. 8 is a plan view of the bottom of a modified form of a field displacement body.

Referring now to the drawings, and particularly to FIG. 1, the coaxial line resonator 2 consists of a coaxial-line section having an outer conductor 5 and an inner conductor 4, which are short-circuited at one end thereof by a conductive plate 6. The inner conductor 4 is constructed so that with an average operational frequency f,- it has a length of M4 where )t is the wavelength of energy at a frequency f,,. To change the resonance frequency, a short-circuited coupling loop 1,

.which is rectangular in form, is disposed at a distance r from the longitudinal axis of the inner conductor 4. The distance r is measured between the central axis of theinner conductor 4 and the axis of symmetry of the loop 1, which is parallel thereto.

The current within the central conductor 4 varies along its length, as well known in the art, and a graph of the current relative to the length of the conductor 4 is illustrated in FIG. 1 by a curve J. The current is greatest at the short circuiting wall 6, and falls to zero at the free end of the inner conductor 4.

The greatest detuning Af of the resonant frequency is attained when the loop 1 is located in the region of the maximum current along the coaxial section, as the magnetic field strength is also the greatest at this location. Accordingly, as the current is greatest in the vicinity of the short-circuiting wall 6, it is desirable to mount the loop 1 adjacent or close to the wall 6. In the embodiment of FIG. I the coupling loop 1 is mounted on a circular shaft 7 having a flange 8 which butts against the outer surface of the wall 6. The shaft 7 is received in a circular aperture of the wall 6, so that it may be rotated therein, thereby rotating the loop I about an axis parallel to the axis of the inner conductor 4. The flange 8 is pressed against the plate 6 by a leaf spring 3 having one end mounted on a bracket 10 secured to the outer surface of the plate 6. Friction between the spring 10 and the flange 8 insures against unintentional rotation of the loop 1 after its position has been selected. If desired, the flange 8 may be provided with one or more notches or grooves which function to simplify the setting of the attitude of the loop 1 to the desired frequency, and if two springs are provided arranged opposite one another for alternately engaging the notches, the number of positions at which the rotation of the shaft 7 can be arrested is doubled in comparison with an arrangement having only one spring.

The mutual inductance M between the loop I and the resonator 2 is varied by rotating the loop 1, via the shaft 7. As a result the resonance frequency f,, is varied by an amount Af.

When the loop is very small, and the resonator has an inner conductor 4 which is a quarter wave length long, M=2 F sin a/r and Af=4 M flFZ L where F is the area of the loop I, r is the distance from the center of the central conductor 4 to the axis of the symmetry of the loop 1 which is parallel thereto, L is the inductance of the loop 1, Z is the surge impedance of the resonator, and a is the angle between the plane of the coupling loop and a plane perpendicular to a radius of the coaxial-line section.

Referring now to FIG. 2, an alternative embodiment is illustrated in which the loop 1 is mounted on the outer conductor rather than on the end wall 6. The elements in FIG. 2 which correspond to elements in FIG. 1 are identified with the same reference numerals. The coupling loop 1 remains in the region of maximum current, near the end wall 6.

The shape of the loop 1 and the cross-section of the loop are not critical for the desired effect of detuning the resonator by rotation of the shaft 7, and may be freely selected within wide limits, e.g. round, square or rectangular. The loop, together with the shaft 7, can be formed of a single piece of metal and are preferably cast of aluminum.

The coupling loop can alternatively consist of a single flat plate as shown in FIGS. 3 and 4. The shaft 7 is provided with a flange 8, as is the apparatus of FIGS. 1 and 2, and the loop itself takes the form of a flat square rectangular plate 11. An aperture 12, shown in broken lines in FIG. 4, can be provided in the plate '11, in which case the loop arrangement of FIGS. 1 and 2 is simulated. The presence of an aperture 12 provides a greater shift in the resonance frequency than if the aperture I2 is eliminated. However, even without an aperture l2 rotation of the shaft 7 effects a shift in the frequency of the resonator. The term field displacement body" is used to designate the plate 11, with or without an aperture 12, and also to designate a loop of the form shown in FIGS. 1 and 2.

Referring now to FIGS. 5-7, another alternative arrangement of the present invention is illustrated. The field displacement body shown in FIGS. 5-7 has, in addition to the plate 11 similar to that shown in FIGS. 3 and 4, an additional field displacement body 13, which is mounted to the inner end of the shaft 7 at an eccentric position. The cross-section of the field displacement body 13 is .preferably eliptical, as illustrated in FIG. 7. When the additional field displacement body 13 is provided, the effective angle of rotation is doubled, in relation to the arrangement with only a single field displacement body.

When the additional field displacement body 13 is provided it is possible for the resonator to be tuned to frequencies which are relatively equidistant from each other, by rotating the shaft 7 by the same angle a in opposite directions from the position in which the frequency of the resonator is at its intermediate value. When the shaft 7 is mounted in the short-circuiting end wall 6, as illustrated in FIG. 1, the central frequency is selected when the plane of the plate 1 l is perpendicular to a radius of the coaxial-line section.

If desired, a third field displacemnt body, is illustrated in dashed lines in FIG. 7, can be provided, or as another alternative, the inner end of the shaft 7 may be provided with two eccentrically mounted field displacement bodies with the central plate 11 eliminated. For any angle a, the detuning Af of the coaxial-line resonator depends upon the number and size of the field displacement bodies. The eliptical cross-section illustrated in FIG. 7 has proved to be a favorable form for eccentric field displacement bodies, but the crosssection may alternatively be circular, oval or angular. The eliptical field displacement body shown in FIG. 7 has its major axis oriented in parallel to the plate 11, with the angle fl therebetween being zero. Alternatively the angle B may have a value other than zero, when the major axis of the eliptical body 13 is rotated relative to the plane of the body 11, as shown in FIG. 8. In this way an increase in the angle of rotation of the shaft 7 is required to produce the same frequency deviation, so that there is in this case a less stringent requirement on the setting accuracy of the shaft 7.

In FIG. 8, an arrangement is shown in which the eliptical field displacement body 13 is positioned with its major axis forming an angle B with the plane of the plate 11. Such an arrangement has proven to be particularly advantageous since it results in improved linearity of the detuning Af as a function of the rotation of the shaft 7. An angle of 10 to 15 for ,8 has been found to give particularly good results. The resonant frequency of the resonator, as a function of the angle a, is modified by varying'the angle B.

It is apparent from the foregoing that, in addition to the advantages of being simple and economical in construction, and being substantially independent of temperature, the present invention also achieves the advantage of making it unnecessary to have any special apparatus for establishing electrical contact between the field displacement body and the parts of the resonator.

What is claimed is:

l. A coaxial line section resonator comprising: a coaxial line section having one end short-circuited by a conductive plate, an electrically conductive field dis placement body supported in the space between the conductors of said coaxial line section near said conductive plate, said field displacement body being formed integrally with a rotatable member extending from the exterior of said resonator into the space between the conductors of said coaxial line section, said field displacement body being a relatively flat body, and a second field displacement body mounted on said rotatable member, the major plane of said second field displacement body being parallel to that of the first field displacement body, said second field displacement body being arranged eccentrically to the axis of rotation of said rotatable member.

2. A resonator as claimed in claim 1, wherein said additional field-displacement body has an eliptical crosssection.

3. A resonator as claimed in claim 2, wherein the major axis of the eliptical cross-section of said additional body makes an angle with the plane of said field displacement element.

4. A coaxial line section resonator tunable by means of field displacement, said resonator comprising a coaxial line section which is short-circuited at one end by a short-circuit plate consisting of electrically conductive material, a rotatable member mounted on said resonator in the vicinity of said short-circuit plate, and a flat field displacement body formed of electrically conductive material mounted on said member, said field displacement body being arranged eccentrically to the axis of rotation of said rotatable member, the location wherein said second field displacement body is arranged eccentrically with said axis of rotation.

7. The coaxial line resonator according to claim 4, wherein at least one of said field displacement bodies has an elliptical cross section.

8. The coaxial line resonator according to claim 4, wherein at least one of said field displacement bodies is provided with an aperture. 

1. A coaxial line section resonator comprising: a coaxial line section having one end short-circuited by a conductive plate, an electrically conductive field displacement body supported in the space between the conductors of said coaxial line section near said conductive plate, said field displacement body being formed integrally with a rotatable member extending from the exterior of said resonator into the space between the conductors of said coaxial line section, said field displacement body being a relatively flat body, and a second field displacement body mounted on said rotatable member, the major plane of said second field displacement body being parallel to that of the first field displacement body, said second field displacement body being arranged eccentrically to the axis of rotation of said rotatable member.
 2. A resonator as claimed in claim 1, wherein said additional field-displacement body has an eliptical cross-section.
 3. A resonator as claimed in claim 2, wherein the major axis of the eliptical cross-section of said additional body makes an angle with the plane of said field displacement element.
 4. A coaxial line section resonator tunable by means of field displacement, said resonator comprising a coaxial line section which is short-circuited at one end by a short-circuit plate consisting of electrically conductive material, a rotatable member mounted on said resonator in the vicinity of said short-circuit plate, and a flat field displacement body formed of electrically conductive material mounted on said member, said field displacement body being arranged eccentrically to the axis of rotation of said rotatable member, the location of said field displacement body within said resonator being dependent on the angle of rotation of said rotatable member, and including a second flat field displacement body mounted on said member parallel to the first field displacement body.
 5. The coaxial line resonator according to claim 4, wherein said second field displacement body is aligned with said axis of rotation.
 6. The coaxial line resonator according to claim 4, wherein said second field displacement body is arranged eccentrically with said axis of rotation.
 7. The coaxial line resonator according to claim 4, wherein at least one of said field displacement bodies has an elliptical cross section.
 8. The coaxial line resonator according to claim 4, wherein at least one of said field displacement bodies is provided with an aperture. 